Method of ultrasonic mounting and ultrasonic mounting apparatus using the same

ABSTRACT

A method of ultrasonic mounting can increase mounting efficiency by using high-frequency ultrasound and can also mount large semiconductor chips. The method ultrasonically bonds a semiconductor chip  52  to a substrate  50  using an ultrasonic mounting apparatus including a horn  15  that propagates ultrasonic vibration of an ultrasonic vibrator, the horn  15  being made of a ceramic that has a higher vibration propagation speed than metal. The method includes steps of disposing the substrate  50  on a stage  13 , disposing the semiconductor chip  52  on the substrate  50 , and placing the semiconductor chip  52  in contact with a convex part  15   a  provided on the horn  15  and applying ultrasonic vibration to bond the semiconductor chip  52  to the substrate  50.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of ultrasonic mounting thatbonds a semiconductor chip to a substrate and an ultrasonic mountingapparatus using the same.

2. Related Art

When flip-chip bonding and mounting a semiconductor chip on a circuitboard, a method is used that places electrode terminals, such as bumps,of the semiconductor chip in contact with electrode terminals, such aspads, of the circuit board and applies ultrasonic vibration to thesemiconductor chip to bond the electrode terminals of the semiconductorchip and the circuit board together.

Conventionally, around 50 kHz is used as the frequency of the ultrasoundapplied to the semiconductor chip.

However, the present inventors have discovered that a large amount ofbonding energy is obtained when ultrasound with a high frequency ofaround 200 kHz is used, which means that the mounting efficiency can beincreased.

During ultrasonic mounting, vibration is applied from an ultrasonicvibrator to the semiconductor chip via a horn, and to transmit a largeamount of vibration energy, the horn is provided with a convex part of arequired width at a position corresponding to the loop (maximumamplitude point) of the vibration caused by the ultrasonic vibration andthe semiconductor chip is placed in contact with this convex part totransmit the vibration energy.

When high-frequency ultrasound is used, there is a correspondingreduction in wavelength. Accordingly, the width of the convex partprovided corresponding to the maximum amplitude point inevitably becomesnarrow, so that there is the new problem that only small semiconductorchips can be mounted.

SUMMARY OF THE INVENTION

The present invention was conceived to solve the problems describedabove, and it is an object of the present invention to provide a methodof ultrasonic mounting and an ultrasonic mounting apparatus using thesame that can increase mounting efficiency by using high-frequencyultrasound and can also mount large semiconductor chips.

A method of ultrasonic mounting according to the present inventionultrasonically bonds a semiconductor chip to a substrate using anultrasonic mounting apparatus in which a horn that propagates ultrasonicvibration of an ultrasonic vibrator is made of a ceramic that has ahigher vibration propagation speed than metal, the method includingsteps of: disposing the substrate on a stage; disposing thesemiconductor chip on the substrate; and placing the semiconductor chipin contact with a convex part provided on the horn and applyingultrasonic vibration to bond the semiconductor chip to the substrate.

An ultrasonic mounting apparatus according to the present inventionincludes a horn for propagating ultrasonic vibration of an ultrasonicvibrator and bonds a semiconductor chip to a substrate by placing thesemiconductor chip in contact with a convex part of the horn andapplying ultrasound, wherein the horn is formed of a ceramic that has ahigher vibration propagation speed than metal.

Stepped parts may be provided in walls of the convex part.

A spacer, which is composed of a material that has a vibrationpropagation speed of an intermediate magnitude between a vibrationpropagation speed of the ultrasonic vibrator which is made of metal anda vibration propagation speed of the horn which is made of ceramic, maybe interposed at a joint between the ultrasonic vibrator and the horn.

A male screw for joining to the horn may be formed on the ultrasonicvibrator and a coating layer composed of a soft metal material such ascopper or solder may be formed on the male screw.

Another method of ultrasonic mounting according to the present inventionultrasonically bonds a semiconductor chip to a substrate using anultrasonic mounting apparatus including a horn that propagatesultrasonic vibration of an ultrasonic vibrator, two convex parts beingformed on the horn corresponding to maximum amplitude points that appearone wavelength apart and vibrate in the same direction due to theultrasonic vibration, the method including steps of: disposing thesubstrate on a stage; disposing the semiconductor chip on the substrate;and inserting the semiconductor chip between the two convex parts of thehorn and applying ultrasonic vibration from the two convex parts to bondthe semiconductor chip to the substrate.

Stepped parts that can be engaged by the edge parts of the semiconductorchip may be formed in walls of the two convex parts that face oneanother, the semiconductor chip may be inserted between the two convexparts so that the edge parts engage the stepped parts, and ultrasoundmay be applied while the semiconductor chip is pressed by surfaces ofthe stepped parts.

The semiconductor chip may be inserted between the two convex parts viaan elastic body.

Another method of ultrasonic mounting according to the present inventionultrasonically bonds a semiconductor chip to a substrate using anultrasonic mounting apparatus including a stage that propagatesultrasonic vibration of an ultrasonic vibrator, two convex parts beingformed on the stage corresponding to maximum amplitude points thatappear one wavelength apart and vibrate in the same direction due to theultrasonic vibration, the method comprising steps of: disposing thesubstrate on the stage so as to be inserted between the two convexparts; disposing the semiconductor chip on the substrate; and applyingultrasonic vibration from the two convex parts to the substrate whilethe semiconductor chip is pressed by a pressing mechanism to bond thesemiconductor chip to the substrate.

Stepped parts that can be engaged by the edge parts of the substrate maybe formed in walls of the two convex parts that face one another and thesubstrate may be inserted between the two convex parts so that the edgeparts engage the stepped parts.

Walls of the two convex parts that face one another may be formed asinclined surfaces and the substrate may be disposed between the twoconvex parts so as to be inserted between the inclined surfaces.

The substrate may be inserted between the two convex parts via anelastic body.

With the method of ultrasonic mounting and ultrasonic mounting apparatusaccording to the present invention, it is possible to increase mountingefficiency by using high-frequency ultrasound and to mount largesemiconductor chips.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the entire construction of amounting apparatus;

FIG. 2 is a diagram useful in further explaining an ultrasonic bondingunit in the mounting apparatus shown in FIG. 1;

FIG. 3 is a diagram useful in explaining the relationship between a hornand an ultrasonic vibrator;

FIG. 4 is an enlarged view of part of FIG. 3;

FIG. 5 is a graph showing the relationship between the vibrationpropagation speed due to the horn material and the effective tool size;

FIGS. 6A and 6B are diagrams useful in explaining a construction wherestepped parts are provided in walls of the convex parts of the horn;

FIGS. 7A and 7B are diagrams useful in explaining warping of the convexparts;

FIG. 8 is a diagram useful in explaining an embodiment where a spacer isinterposed between the horn and the ultrasonic vibrator;

FIG. 9 is a diagram useful in explaining an embodiment where a coatinglayer is formed on a male screw of the ultrasonic vibrator;

FIG. 10 is a diagram useful in explaining an embodiment where two convexparts are provided on the horn;

FIG. 11 is a diagram useful in explaining an embodiment where inclinedsurfaces are provided on the convex parts;

FIG. 12 is a diagram useful in explaining an embodiment where thesemiconductor chip is inserted via an elastic body;

FIG. 13 is a diagram useful in explaining an embodiment where two convexparts are provided on the stage;

FIG. 14 is a diagram useful in explaining an embodiment where inclinedsurfaces are provided on the convex parts; and

FIG. 15 is a diagram useful in explaining an embodiment where thesubstrate is inserted via an elastic body.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail with reference to the attached drawings.

FIG. 1 is a schematic diagram showing one example of the entireconstruction of a flip-chip mounting apparatus 10.

Reference numeral 12 designates an ultrasonic bonding unit. Theultrasonic bonding unit 12 includes a stage 13 onto which a substrate isconveyed and a bonding tool 14 that is disposed above the stage 13,holds a semiconductor chip on a lower surface thereof, and can moverelatively toward and away from the stage 13.

The stage 13 is composed of a well-known XY table and can be moved in adesired direction within a horizontal plane by a driving unit, notshown. The XY table is constructed so as to be capable of being rotatedwithin the horizontal plane about the vertical axis by a rotationaldriving unit, not shown.

The bonding tool 14 is composed of a well-known ultrasonic bondingdevice, and includes a horn 15 for ultrasonic bonding and a pressingdevice 16 that is composed of a cylinder mechanism or the like thatmoves the horn 15 up and down. A semiconductor chip is held on a lowersurface of the horn 15 by suction.

A camera device 18 for position recognition is disposed so as to becapable of insertion between the stage 13 and the bonding tool 14. Thecamera device 18 detects the positions of a substrate conveyed onto thestage 13 and a semiconductor chip held on the horn 15 of the bondingtool 14, and aligns the substrate and the semiconductor chip byhorizontally moving the stage 13 and/or rotating the stage 13 within thehorizontal plane.

FIG. 2 is a diagram useful in further explaining the ultrasonic bondingunit 12. The ultrasonic bonding unit 12 is a well-known mechanism andtherefore will be described in brief.

Reference numeral 20 designates a pressing force control unit thatcontrols the pressing device 16, 21 an ultrasonic vibrator, 22 an imageprocessing unit, 23 a moving device that moves the camera device 18, 24a movement control unit that controls movement by the moving device 23,25 an alignment control unit that controls movement and rotation of thestage 13, and 26 a main controller.

By driving the moving device 23 using the movement control unit 24, thecamera device 18 is inserted between the substrate that has beenconveyed onto the stage 13 and the semiconductor chip that is held onthe horn 15 by suction. Image data from the camera device 18 is inputtedinto the image processing unit 22, positional displacements between thesubstrate and the semiconductor chip are detected, and the stage 13 ismoved and/or rotated by the alignment control unit 25 to correct anypositional displacements, thereby aligning the substrate and thesemiconductor chip. Next, the camera device 18 is withdrawn. After this,the pressing device 16 is driven by the pressing force control unit 20to lower the horn 15 and apply a predetermined force to thesemiconductor chip held on the lower surface of the horn 15 andultrasound is applied from the ultrasonic vibrator 21 to thesemiconductor chip to bond the semiconductor chip to the substrate.Driving control of the various control units is entirely carried out bya processing program set in the main controller 26.

In FIG. 1, reference numeral 35 designates a conveying unit forsemiconductor chips.

A large number of semiconductor chips are stored on a tray (not shown)and are supplied by a chip supplying stage 36. Using a chip handler 38that includes the suction nozzle 37 that can move up and down andhorizontally, the semiconductor chips stored in the tray are held one ata time by suction on the suction nozzle 37 and are conveyed onto amounting table 41 of a chip inverting stage 40.

The chip inverting stage 40 has a suction arm 42. The suction arm 42includes a suction nozzle 43 and is provided so as to be capable ofbeing inverted by 180° by an inverting device 44 between a positionlocated above the mounting table 41 and a position on an opposite side.The inverting device 44 is also provided so as to be capable of beingmoved back and forth by a driving unit, not shown, in a direction thatapproaches the mounting table 41 and a direction that approaches thehorn 15.

The semiconductor chip is conveyed onto the mounting table 41 with asurface on which bumps are formed facing upwards. By holding thesemiconductor chip conveyed onto the mounting table 41 by suction on thesuction nozzle 43 of the suction arm 42, inverting the suction arm 42,and moving the semiconductor chip towards the horn 15, the semiconductorcan be held on the lower surface of the horn 15 by suction. Thesemiconductor chip therefore becomes held by suction on the horn 15 withthe surface on which the bumps are formed facing downwards.

It should be noted that the suction nozzle 43 is provided so as to becapable of being inwardly and outwardly projected (moved) by amechanism, not shown, in a direction perpendicular to the suction arm 42so that a semiconductor chip can be smoothly transferred between themounting table 41 and the horn 15.

The substrate is conveyed onto the stage 13 by a substrate conveyor orthe like, not shown.

On the other hand, as described above, a semiconductor chip 52 isconveyed into the ultrasonic bonding unit 12 by the conveying unit 35for semiconductor chips and is held by suction on the lower surface ofthe horn 15.

The camera device 18 is inserted between the substrate 50 conveyed ontothe stage 13 and the semiconductor chip 52 held on the horn 15 andalignment of the substrate 50 and the semiconductor chip 52 is carriedout as described above.

Next, the camera device 18 is withdrawn and the horn 15 on which thesemiconductor chip 52 is held by suction is lowered by the pressingdevice 16 so that the semiconductor chip 52 is pressed onto thesubstrate 50 with the required pressing force. After this, theultrasonic vibrator 21 is operated and ultrasound is applied to thesemiconductor chip 52 from the horn 15. By doing so, bumps 52 a of thesemiconductor chip 52 are ultrasonically bonded to pads (not shown) ofthe substrate 50.

FIG. 3 shows the relationship between the horn 15, the ultrasonicvibrator 17, the semiconductor chip 52, and the substrate 50 in theflip-chip mounting apparatus 10 described above. FIG. 4 is an enlargedview of part of FIG. 3. Convex parts 15 a are formed on an upper surfaceand a lower surface of the horn 15.

The ultrasonic vibration propagates as compressional waves inside thehorn 15. In this case, in principle, loops (maximum amplitude points)occur at both ends of the horn 15 and a plurality of other maximumamplitude points occur in intermediate part of the horn 15. The convexparts 15 a are formed with a required width at a position correspondingto such a maximum amplitude point.

Such maximum amplitude points for the ultrasonic vibration naturallyoccur at intervals of one half of the wavelength.

The positions of the maximum amplitude points of the compressional wavesare positions at which the maximum vibration in the horizontal directioncan be applied from the horn 15 to the semiconductor chip 52 and arepositions where the ultrasonic energy can be transmitted to the greatestpossible extent, and by providing the convex parts 15 a of the requiredwidth at such positions, it is possible to carry out ultrasonic bondingof the semiconductor chip 52 efficiently. The width of the convex parts15 a extends across a maximum amplitude point and corresponds to a rangewhere a substantially uniform amplitude value is obtained.

The ultrasonic vibrator 17 is composed of metal, such as a titaniumalloy, in which a piezoelectric element is incorporated.

As in a conventional device, the horn 15 is formed of metal such astitanium alloy.

The speed at which ultrasound propagates within a member is unique tothe member, and is determined by the material used.

However, the relationship between the propagation speed C, the frequencyf, and the wavelength λ is C=f□λ.

Accordingly, when the frequency is changed from 50 kHz to a highfrequency of 200 kHz, the wavelength is quartered. Conventionally, ifthe horn 15 is formed of a metal such as a titanium alloy, when thefrequency is 50 kHz, it is possible to set the width of the convex parts15 a at around 12 mm and bonding can be carried out for semiconductorchips that are around 12 mm in size. However, when the frequency israised to 200 kHz, the wavelength is quartered so that the width of theconvex parts 15 a is also reduced to around one quarter, that is, thewidth can be set at only around 3 to 4 mm, so that large semiconductorchips can no longer be mounted.

However, in the present embodiment, the horn 15 to which the ultrasonicvibration of the ultrasonic vibrator is propagated is formed of aceramic that has a high vibration propagation speed compared to metal.

The vibration propagation speed (m/sec) of various metals and ceramicsare shown below.

Iron 5,950 A5052 6,190 Titanium Alloy 6,313 (Ti—6Al—4V Alloy) Zirconia7,036 Cermet 9,086 Aluminum Nitride 10,198 Silicon Nitride 10,764 Sialon11,032 Alumina 11,804 Silicon Carbide 12,018 Single Crystal Sapphire12,624

For the present invention, the expression “ceramics” includes zirconiaand cermet. Aside from these, ceramics such as mullite, titaniaceramics, and cordierite are effective.

As described above, when ceramics are used, the vibration propagationspeed is around double that of metal, and accordingly even when highfrequency ultrasound with a frequency of 200 kHz is used, the width ofthe convex parts 15 a of the horn 15 can be increased to around 8 mm, sothat even large semiconductor chips can be mounted.

FIG. 5 is a graph showing a model of the relationship between variousmaterials of the horn and the vibration propagation speed and effectivetool size (the width of the convex parts). It can therefore beunderstood that the tool size (the width of the convex parts) can beincreased by using ceramics as the material of the horn 15.

FIGS. 6A and 6B show an embodiment where stepped parts 19 are providedin the walled parts of the convex parts 15 a of the horn 15 describedabove.

In FIGS. 7A and 7B, the convex parts 15 a are simply provided on thehorn 15, but due to the provision of the convex parts 15 a, an amplitudecomponent is produced in a height direction (Z direction) of the convexparts 15 a and the convex parts 15 a deform to become warped (see FIG.7B), so that there is the problem that ultrasonic vibration cannot betransmitted uniformly to the semiconductor chip 52.

As shown in FIG. 6B, by providing stepped parts 19 in the walls of theconvex parts 15 a, there is the effect that warping is absorbed by thestepped parts 19 and the overall warping of the convex parts 15 a isreduced.

FIG. 8 shows an embodiment in which a spacer 27 is interposed at a jointof the horn 15 and the ultrasonic vibrator 17.

A material that has a vibration propagation speed of an intermediatemagnitude between the vibration propagation speed of the ultrasonicvibrator 17 that is made of metal and the vibration propagation speed ofthe horn 15 that is made of ceramic is used for this spacer 27.

As one example, titanium alloy is used as the ultrasonic vibrator 17,cermet is used as the spacer 27, and alumina is used as the horn 15.

When the ultrasonic vibrator 17 is made of metal and the horn 15 is madeof ceramic, there is a large difference in vibration propagation speeddue to these materials, so that there is the risk of the ultrasonicvibration being reflected at the interface of the ultrasonic vibrator 17and the horn 15 which reduces the transmissibility of the ultrasound,but this problem can be solved by interposing the spacer 27 that has anintermediate vibration propagation speed relative to the two parts.

FIG. 9 shows an embodiment in which a coating layer composed of a softmetal material such as copper or solder is formed on a surface of a malescrew 17 a of the ultrasonic vibrator 17 used for joining the horn 15.

The ultrasonic vibrator 17 is integrated by screwing the male screw 17 ainto a female screw thread (not shown) of the horn 15, and by providinga coating layer of a soft metal material on the surface of the malescrew 17 a, gaps between the two parts are filled when the differentmaterials are screwed together and the different materials can beconnected so as to fit together well.

FIG. 10 shows yet another embodiment.

In this embodiment, a component with two convex parts 15 a, 15 b, whichcorrespond to maximum amplitude points P that appear at an interval ofone wavelength and vibrate in the same direction due to the ultrasonicvibration, is used as the horn 15.

By disposing the substrate 50 on the stage 13, disposing thesemiconductor chip 52 on the substrate 50, inserting the semiconductorchip 52 between the two convex parts 15 a, 15 b of the horn 15, andapplying ultrasonic vibration from the two convex parts 15 a, 15 b, thesemiconductor chip 52 is bonded to the substrate 50.

Here, the rear surface of the semiconductor chip 52 is set so as to notcontact the horn 15. Since the compressional waves propagate so that theconvex parts 15 a, 15 b vibrate in synchronization (i.e., with the samephase), there is no risk of the semiconductor chip 52 being destroyed.

Also, since the semiconductor chip 52 is inserted between the two convexparts 15 a, 15 b that are not half but one wavelength apart, mountingcan be carried out for large semiconductor chips. The material of thehorn 15 is not limited to ceramics, and a metal horn may be used.

In the embodiment shown in FIG. 11, inclined surfaces 28 are formed atthe walls of the convex parts 15 a, 15 b shown in FIG. 10 that face oneanother, with the semiconductor chip 52 being disposed in the two convexparts 15 a, 15 b so as to be inserted between the inclined surfaces 28.By doing so, it is possible to apply ultrasound while pressing thesemiconductor chip 52 via the inclined surfaces 28 onto the substrate 50with a predetermined pressing force. It should be noted that in place ofthe inclined surfaces 28, stepped parts (not shown) that can engage edgeparts of the semiconductor chip 52 can be formed, with the semiconductorchip 52 being inserted between the two convex parts 15 a, 15 b so thatthe edge parts engage the stepped parts and with ultrasound beingapplied while the semiconductor chip 52 is pressed by the steppedsurfaces.

In the embodiment shown in FIG. 12, the semiconductor chip 52 isinserted between the convex parts 15 a, 15 b shown in FIG. 10 via anelastic body 29. Ultrasound can be applied to the semiconductor chip 52from the convex parts 15 a, 15 b via the elastic body 29.

FIG. 13 shows a construction where instead of applying ultrasound from ahorn, the ultrasonic vibrator 17 is attached to the stage 13 andultrasonic vibration is applied to the substrate 50 disposed on thestage 13 to bond the semiconductor chip 52.

In this embodiment, two convex parts 13 a, 13 b corresponding to maximumamplitude points P that appear one wavelength apart and vibrate in thesame direction due to the ultrasonic vibration are formed in the stage13.

The substrate 50 is disposed on the stage 13 so as to be insertedbetween the two convex parts 13 a, 13 b, the semiconductor chip 52 isdisposed on the substrate 50, and while the semiconductor chip 52 ispressed by an appropriate pressing mechanism (not shown), ultrasonicvibration is applied to the substrate 50 from the two convex parts 13 a,13 b so as to bond the semiconductor chip 52 onto the substrate 50.

In the embodiment shown in FIG. 14, inclined surfaces 28 are formed atthe walls of the convex parts 13 a, 13 b shown in FIG. 13 that face oneanother, with the substrate 50 being disposed between the two convexparts 13 a, 13 b so as to be inserted between the inclined surfaces 28.It is therefore possible to apply ultrasound to the substrate 50 via theinclined surfaces 28. It should be noted that in place of the inclinedsurfaces 28, stepped parts (not shown) that can be engaged by edge partsof the substrate 50 can be formed, with the substrate 50 being insertedbetween the two convex parts 13 a, 13 b so that the edge parts engagethe stepped parts.

In the embodiment shown in FIG. 15, the substrate 50 is inserted betweenthe convex parts 13 a, 13 b shown in FIG. 13 via an elastic body 29.Ultrasound can be applied to the substrate 50 from the convex parts 13a, 13 b via this elastic body 29.

1. An ultrasonic mounting apparatus that includes a horn for propagatingultrasonic vibration of an ultrasonic vibrator and that bonds asemiconductor chip to a substrate by placing the semiconductor chip incontact with a convex part of the horn and applying ultrasound, whereinthe horn is formed of a ceramic that has a higher vibration propagationspeed than metal.
 2. An ultrasonic mounting apparatus according to claim1, wherein stepped parts are provided in walls of the convex part.
 3. Anultrasonic mounting apparatus according to claim 1, wherein a spacer,which is composed of a material that has a vibration propagation speedof an intermediate magnitude in between a vibration propagation speed ofthe ultrasonic vibrator which is made of metal and a vibrationpropagation speed of the horn which is made of ceramic, is interposed ata joint between the ultrasonic vibrator and the horn.
 4. An ultrasonicmounting apparatus according to claim 1, wherein a male screw forjoining to the horn is formed on the ultrasonic vibrator and a coatinglayer composed of a soft metal material such as copper or solder isformed on the male screw.